1. Field of Use
Any medium containing electrically conductive material can be monitored for changes in the electrical resistivity of the electrically conductive material. An important and well-known example is the measurement of the electrical resistivity of underground geologic formations. Such formations often contain salt water, which is a relatively high electrically conductive material and/or hydrocarbons which are less electrically conductive and therefore more electrically resistive. The resistivity of geologic formations containing these materials is measurable, thereby providing information relative to the concentrations of oil, water and gas at various locations within a geologic formation.
It has been demonstrated that it is possible to measure the resistivity of an underground geologic formation through a ferromagnetic well casing penetrating the formation, as well as through other materials that are electrically conductive and magnetically permeable. These materials are referred herein as xe2x80x9cEM barriersxe2x80x9d or xe2x80x9cbarrier materials.xe2x80x9d The technology is based upon inductive magnetic coupling; therefore the measuring apparatus does not need to be in physical contact to the barrier material comprising the well casing or production tubing. It has also been shown that magnetic flux may be transmitted through metals that have a permeability of one weber/amp. These metals can be considered non-permeable, i.e., there is little or no absorption of magnetic flux. However, due to the rapid spreading of the magnetic flux, the flux intensity drops off with respect to distance as the inverse cube of the distance.
The invention subject of this specification utilizes the technique of inductive magnetic coupling of Electromagnetic waves to EM barrier materials in combination with transmission of Electromagnetic waves through non-permeable material to facilitate the measurement of resistivity of geologic formation beyond the well casing.
Hydrocarbon production wells typically utilize an outer casing made of a ferromagnetic material. Common outside diameters of the casing may be of a range of 7 to 10 inches or larger. The interior diameter is of varying dimensions. The thickness of the casing also varies but may typically be xc2xd inch or more in thickness. Placed inside the permanent casing is a smaller production tubing. The outer diameter of the production tubing may be in a range of 1 to 4 inches. Hydrocarbon, such as crude oil mixed with salt water or solid particles such as sand, flows through the production tubing at a high velocity. This environment is harsh and corrosive, sometimes requiring the replacement of the production tubing. It will be appreciated that well casing is required to maintain an open channel for the flow of production and minimize interruption due to a xe2x80x9ccave inxe2x80x9d or crumbling of the bore hole wall.
Note that throughout this specification, the terms xe2x80x9ccasing,xe2x80x9d xe2x80x9cwell bore casingxe2x80x9d and xe2x80x9cwell casingxe2x80x9d will be deemed to include hydrocarbon production tubing or other ancillary structures such as casing or tubing connectors, collars or couplings.
The sensor tool of this invention may create a xe2x80x9cMetallic Transparencyxe2x80x9d(trademark) region local to its oscillating magnetic flux transmitter (transmitter) or its flux receiver (receiver) by means of a strong magnetic flux field saturating an EM barrier near the transmitter and receiver. It may also utilize near practical saturation and xe2x80x9cMagnetic Lensxe2x80x9d(trademark) focus to direct the oscillating flux of the transmitter in a controlled manner.
The present invention relates generally to measuring resistivity of media such as liquids, gases or other objects within a geologic formation surrounding a well casing. Specifically, the present invention relates to through casing resistivity measurement in downhole hydrocarbon production environments. The present invention provides a sensor apparatus and method for measuring the resistivity of a formation proximate to a well. The current invention, in one embodiment, is an apparatus for measuring the resistivity of the surrounding formation. The apparatus records electrical responses corresponding to magnetic fluxes that relate to the resistivity of the geologic formation at various depths or locations within the formation penetrated by the cased well. The apparatus can thereby detect the location and amplitude of said resistivity in single or multiple directions, and at distances that will help operators of wells adjust their production management and their reservoir management activities. The apparatus can also be used to detect changes in the resistivity over time by comparison of recorded fluxes at various time intervals.
The invention includes a sensor tool that can travel through the relatively narrow diameter of well casing (or production tubing), means to raise and lower the tool to a specified location of the casing string proximate to a non magnetically permeable section, means to supply power and receive data, as well as recording and display devices.
The invention subject of this specification allows the transmission of Electromagnetic waves through well casing, thereby permitting information of the lithology to be obtained from sensors within the well. Recent developments have shown that magnetic flux may be transmitted though ferromagnetic or paramagnetic materials, such as carbon steel. Ferromagnetic metals or paramagnetic metals, being electrically conductive and magnetically permeable have previously been barriers to the transmission of Electromagnetic energy.
2. Summary of Related Art
In the development and production of oil and gas reservoirs, there is a very significant need to increase the amount and accuracy of information regarding the composition and changes in the composition of the resource-bearing zones of the formation. Resistivity measurement has long been used to characterize properties of the immediately surrounding substrates prior to the inception of production. However, it has typically only been possible to take such measurements prior to setting casing or while the formation itself is otherwise xe2x80x9cexposedxe2x80x9d to a logging tool, i.e., an xe2x80x9copen holexe2x80x9d without an interceding material that acts as a barrier between the logging tool and the formation substrate. Existing methods of measuring the resistivity of the media within a geologic formation have therefore required that the measurements be taken with logging tools deployed prior to commencement of actual production. After casing is placed in the well and production is underway, it is generally not possible to measure the resistivity of the surrounding geologic formation without interruption of the well production and penetration or removal of the well casing.
As is known to those skilled in the industry, the electrical resistivity of a geologic formation varies as a result of (among other reasons) the changing proportion of hydrocarbon to water contained within the formation. Having the ability to measure at selected locations and directions over time would provide for the unique ability to monitor, for example, the change in the percentage of water versus either oil, gas, or other electrically conductive materials approaching the well, far in advance of such change in fluids actually entering the well. The benefits of such measurements include the ability to see changes in the composition of the formation, i.e., hydrocarbon and water by measuring changes in the resistivity of the formation through the barrier material comprising the well casing.
Numerous attempts have been made to provide an apparatus or method for measuring the electrical resistivity of the surrounding geologic formation through a well casing. See for example U.S. Pat. No. 5,654,639 entitled xe2x80x9cInduction Measuring Devices in the Presence of Metal Wall,xe2x80x9d but requiring an electric current to be passed into the metal pipe wall. The contact device is then disengaged from the wall when the apparatus is moved. Also U.S. Pat. No. 5,426,367, entitled xe2x80x9cLogging of Cased Well by Induction Logging to Plot an Induction Log of the Wellxe2x80x9d and stating the device xe2x80x9cneeds the most intimate contact with the pipe in order to eliminate xe2x80x98air gapsxe2x80x99 and still maintain mechanical integrity.xe2x80x9d Column 8, Lines 40-43. U.S. Pat. No. 6,157,195, entitled xe2x80x9cFormation Resistivity Measurements from within a Cased Well Used to Quantitatively Determine the amount of Oil and Gas Present,xe2x80x9d also requires the transmission of an ac current through the well casing to a remote electrode. This is consistent with earlier patents such as U.S. Pat. No. 6,025,721, entitled xe2x80x9cDetermining Resistivity of a Formation Adjacent to a Borehole Having casing by Generating Constant Current Flow in Portion of Casing and Using at Least Two Voltage Measurement Electrodes,xe2x80x9d U.S. Pat. No. 5,260,661, entitled xe2x80x9cCalibrating and Compensating Influence of Casing Thickness Variations on Measurements of Low Frequency AC Magnetic Fields within Cased Boreholes to Determine Properties of Geological Formations,xe2x80x9d U.S. Pat. No. 5,065,100, entitled xe2x80x9cMeasurement of In-phase and Out-of Phase Components of Low Frequency AC Magnetic Fields within Cased Boreholes to Measure Geophysical Properties of Geological Formations,xe2x80x9d and U.S. Pat. No. 5,038,107, entitled xe2x80x9c Method and Apparatus for Making Induction Measurements Through Casing.xe2x80x9d
Many of these methods have relied upon the transmission of an electrical current through the casing and into the surrounding formation. All of the methods have required electrical contact be maintained between the apparatus and the casing.
There has always been a need to provide the capability for continuous or periodic measurements of formation resistivity for a hydrocarbon production well installation. Specifically, there has been a need to xe2x80x9csee throughxe2x80x9d the well casing to the geologic formation located around the production well. There also has long been a need to provide resistivity measurements without interruption of the hydrocarbon production well. There is a great need to collect the above mentioned data using the innovative and revolutionary method comprising the ability to simultaneously (i) generate a magnetic flux, by conventional means, within the confines of a hydrocarbon production well, (ii) create Metallic Transparency region within or through the well casing of a hydrocarbon production well, (iii) engage or transmit through the transparency with oscillating magnetic flux, and (iv) receive and measure through the well casing magnetic flux that may be generated in the media in the geologic formation outside of the well casing. It is already known to those skilled in the art that such measurements can provide information about the resistivity of media within the formation, and hence the composition or change in the media within the geologic formation proximate to the well.
It has been shown that magnetic flux may be transmitted through metals that have a permeability of one weber/amp. These metals can be considered non-permeable, i.e., there is little or no absorption of magnetic flux. However, due to the rapid spreading of the magnetic flux, the flux intensity drops off with respect to distance as the inverse cube of the distance. This can be expressed as 1/D3 where xe2x80x9cDxe2x80x9d represents the distance between the device and the target object. This means that, in the absence of the Magnetic Lens(trademark) effect, there is a significant geometric limitation of traditional devices attempting to transmit and receive electromagnetic signals through non-permeable metals. In addition, the absence of the Magnetic Lens(trademark) effect or the xe2x80x9cDirected Magnetic Beamxe2x80x9d effect means that the transmitter/receiver (Tx/Rx) separation determines the distance through the metal that will be detected. Assuming the casing material is non-permeable like stainless steel existing as a collar on well casing, a section of well casing or as a flange on a pipeline or any other similar application, the combined 1/D3 phenomena and the transmitter/receiver separation distance are major limitations to the detection range, i.e., distance, of the device. This transmitter and receiver (xe2x80x9cTx/Rxxe2x80x9d) flux field is shown in FIG. 33.
Low frequency electromagnetic waves (2 KHz to 10 KHz) readily pass through non-permeable metals such as stainless steel, and aluminum. The relationship of frequency that may be used to penetrate a particular metal of some thickness is governed by the xe2x80x9cskin depthxe2x80x9d penetration equation. There is a loss in signal intensity because the electrical conductivity of the metal causes a dissipation of electromagnetic energy. However, it is possible to transmit a magnetic wave of sufficient power and sufficiently low frequency to penetrate any non-permeable metal. It is thus possible to design sensors to be placed in sections of well casing or pipelines at which the permeability of the metal is near unity, i.e., 1 weber/amp. These sections could be well casing collars or spool piece inserts between two pipeline flanges.
The invention subject of this specification provides a method and apparatus for measuring the electrical resistivity of the geologic formation proximate to a hydrocarbon production well installation. A measuring device (xe2x80x9csensorxe2x80x9d) is configured to allow it to be placed proximate to i.e., within a portion of well casing. Well casing (including hydrocarbon production tubing) is commonly manufactured of materials that are electrically conductive and magnetically permeable. These materials are referred herein as xe2x80x9cEM barriersxe2x80x9d or xe2x80x9cbarrier materials.xe2x80x9d The present invention may create at least one Metallic Transparency region within an EM barrier material. A Metallic Transparency region permits oscillating magnetic flux to be transmitted through the EM barrier.
The sensor tool of the invention therefore includes the capability of generating magnetic flux to magnetically engage and saturate an EM barrier material (utilizing a xe2x80x9cmagnetic saturation generatorxe2x80x9d), transmitting oscillating magnetic flux into or through the transparency and a portion of the well casing comprised of non-permeable material, measuring any oscillating magnetic flux generated by the eddy currents induced within an electrically conductive material existing within the geologic formation, e.g., water or hydrocarbons. It will be readily understood that the transmission of oscillating magnetic flux into an electrically conductive material will induce an electric current, i.e., eddy current, by well known scientific principles. The present invention also provides the ability to perform these activities continuously thereby sustaining the eddy currents.
By altering the concentration of the saturation flux, the frequency of the transmitter flux, placement of the transmitters and receivers, or by the orientation of the transmitter in relation to the saturation coil, it is possible to vary the depth of penetration into the geologic formation, thus building a detailed characterization profile of the formation at various distances from the casing. The Metallic Transparency region may also be used to directionally control the flux transmitted through the casing and into the surrounding geologic formation. This may be accomplished by the Magnetic Lens(trademark) focus. Furthermore, the horizontal movement of a logging tool will provide profile data used to triangulate depth field information to various zones of significance.
There is a plurality of subsystems that may be incorporated into the invention. These include the following:
Magnetically Non-Permeable Casing System
Full Saturation Magnetic Flux Circuit
Near Practical Saturation Magnetic Flux Circuit
Transmitter/Receiver System
Nulling System-geometric, electronic, permeability
Automatic Lensing System
Conductivity/Resistivity Measurement System
Wall Thickness Measurement System
Each of these subsystems may be incorporated into embodiments of the sensor tool. Each will be discussed as part of the subject invention.